Electrical oscillator having openended coaxial resonator



March 18, 1952 w. G. SHEPHERD 2,589,739

ELECTRICAL OSCILLATQR HAVING OPEN-ENDED COAXIAL RESONATOR Filed Aug. 27, 1947 3 Sheets-Sheet 1 71111111111. rrlzlr.. I

v1/EN To/:f` v W. G. SHEPHERD Bywmmm March 18, 1952 w. G. SHEPHERD 2,589,739

ELECTRICAL OSCILLATOR HAVING OPEN-ENDED COAXIAL RESONATOR Filed Aug. 27, 1947 3 Sheets-Sheet 2 A7' TORNEV March 18, 1952 w, G. SHEPHERD ELECTRICAL OSCILLATOR HAVING OPEN-ENDED COAXIAL RESONATOR 3 Sheets-Sheet 3 Filed Aug. 27, 1947 /N VEN TOR W. G. SHEPHERD MYMMWGL ATTORNEY Patented 'Main 18, 1952 ELECTRICAL osoILLA'ro'ltHAt/ING 'OPEN- E-NDED ooAXIAL mrsoNA'rorry William (i.r4 Shepherd; Summit, N.' J.,l assigner-to. Bell Telephone Laboratories;Incorporated, New York, N.v Y., a. corporationof` Newl York- ApplicationiA-gust 27, 1947; Serial No.770,853-- This invention relates to'microwave resonators and more particularly to the' application of such resonators as frequency-determining components cf. microwave oscillators.

An object of the invention is to increase the size of the physical structure of a microwave resonator.

Another object of the .invention is'to fenablethe use of resonators of conveniently large physical dimensions as the frequency-determining components of microwave electrical oscillators.

An additional object of the invention is to facilitate the use of reflex type oscillators at .high microwave frequencies.

A very useful typev of` microwave oscillator is the so-called reflex 'oscillator disclosed by J. R. Pierce at page 112'of theV Proceedings of the I. R. E. for February 1945, volume 35, N-o. 2. As thel frequencyfofsuchl oscillators is increasedy it becomesf increasingly difficult physically to construct'the'necessary resonant circuits.A An at-Y tempt has been lmade to overcome thisv difficulty by 'employing an harmonic mode` ofoscillation of a relatively large cavity resonator'which has a fundamental mode of resonance of considerably lower frequency than'thatfof the desired oscillations. However, with reflex oscillators having such larger frequencydeterminingresonators another diicultyds-encountered in that oscillators cf the reflex type tend to oscill'ate-at the: gravest or most fundamental mode' of thefreso'j nator; This tends to-'make operation'atthedel siredA higher frequency unstable.'

In accordance with thefinventi'on ahighnQ- resonator is' constructed as a coaxial 4str-ucturelof and the electromagneticleld of the resonatorat anfinteracti'on gapV in 'the tube. TheA reflex' tube' rnay'fb'er so designed as 4-to'maintain its internal! capacitances relatively low.- Couplingiof the- `ifi-- teraction 'gapof "the `-tube to thefresonator:adds` a =capacitive loading `th'eretcrthus 'requiring that theuesona-torfbeJ somewhat physically? short'ene'r'i tno-compensate forthefcapacitive loading: Howey ever,- the resultant resonant structure S will; still4 be 'substantiallyla' quarter.A wavelengthilongerthani wouldza quarter wavelength; structure vi short-:cireV cuitedlat'ioneterminaLL There; saaitendency: for :the coaxial-.resonator;

to'zradiatefenergy:fomf'the high impedanceselec-:

trically 'open "terminal remoterfrom 'the-electroninteraction Lspacei. To' preclude'this theH outer-V cylindrica1..;member' of;- theA coaxial resonator? is extendedlbeyondfthe inner; conductorv to v`consti.- tu-tezf al '.fnubular wave f guide.V

termined that the guide has a low frequency cutoff at a frequency substantially' above that of the oscillations corresponding to the natural resonance frequency of the coaxial section. Under"A these circumstances energy -of the desired Hoscil-4 lation frequency may not 'escape from" the: reso"- natorthrough' the tub'ularwave guide;

The basic oscillation frequency is primarilyi'd'eterm-ined by:` the-electrical dimensions ofthe resonator as modified by thelassociated electron discharge device. A lesser effect in frequencyde= termination is exercised by the transit time of the electrons in their coursein the retarding eld. Variation of the frequency may -be effected by a dielectricmtuner movableinto `the space 'between thecoaXial conductors ofthe resonator sectionto` vary4 the natural-.resonancefrequency of thecoeaxialresonator.

In f-the ldrawing:

Fig. 1 shows schematically a half wavelengthy coaxial resonator open electrically at each end;

Fig. 2 indicates the diminution in length required when a loading capacitance is connected across; the structure near one end;

3shows an extensionV of tthe'outertubular member of fFig. 2 to providel a waveguide; Y

Fig. 4zis a schematic structural and circuitdiagramA of an embodiment of the invention in a .ref

flex oscillator;

Fig.` 5. illustrates a modification-ofthe oscill latorvofeEig.; 4.;

length resonator.open-circuitede at both ends.. Such a resonator is` twice aslong-.as vthe vvcusto-v mary quarter wavelength resonator short-circuited at one end. Ittherefore tends toward a nal structure of considerably larger dimensions than .the .quarter wavelength resonator; and this increase-in structuraldimension is particularlyV advantageous when .dealingwith microwave oscillations; of high A frequenciesfand under; circumstancesrwherei. it is :necessary toI couple the-.reso nator with an electron dishargetube.

Fig. 2 indicates 'the effect/'upon the structure Moreover.v the di.` ameter of the tubular wavevv guidefisso` prede--y of Fig. l when it becomes necessary to load the tor to a dimension 'y somewhat less than A/ 2 but still very much greater than \/4.

Fig. 3 illustrates diagrammatically the modification of the structure of Fig. 2 by extending the outer coaxial conductor il beyond the inner coaxial conductor l2 by a dimension s at the same time giving the extended portion i3 such a diametral dimension as Will cause the extended portion acting as a wave guide to presenta transmission characteristic of the high pass-band lter type such as to prevent escape therethrough from the open end of the coaxial resoonator Ii, i2 of oscillation energy of the natural resonance frequency of that resonator.

Assuming in Fig. l a coaxial transmissionv line with ideal open circuits at each end the Wavelength measured along the coaxial structure is determined by the equation }.=c-J;=./ 2z., o)

wherey k is the wavelength in the coaxial C1 is the wave propagation velocity in the coaxial yo is the physical length of the coaxial section,

- and e is the dielectric constant of the dielectric medium between the inner and outer conductors. Hence If r1 and r2 are respectively the inner and outer radii of the coaxial conductors, r1 being measured as an external dimension and r2 as an internal dimension, and if one is to insure that only the fundamental mode of oscillation is to occur In the case of Fig. 2, the length dimension 'yo of the resonator of Fig. 1 must be changed to some shorter dimension y to compensate for the loading effect of the capacitance C which will be a function of the design and electrical dimensions of the coupled electronic device.

' Assuming that the coaxial resonator has been so designed as to compensate for the coupled tube there remains the problem of excessive energyvradiaton fromthe electrically open end ofthe resonator remote from the electron tube. Fig. 3 presents the solution of this problem by an extension i3 of the outer tubular coaxial member for a distance s thus constituting a hollow tube waveguide extending beyond the inner coaxial member I2. The cut-off of the hollow tubular wave guide may be determined by tor,- e is the dielectric constant of the dielectric medium within the wave guide, and azzrz the radius of the wave guide section.

Equation 4 and the following equations are based on the condition in which the space between the coaxial conductors contains no dielectric material. They may be assumed to apply also where the amount of the dielectric material is small so that its eect may be neglected. Where substantial amounts of dielectric material are employed allowance must be made for the dielectric material and each configuration requires a special design. The principles involved, however, remain the same. The presence of a solid dielectric in the cut-01T wave guide makes it necessary to reduce the diameter of the wave guide to insure cut-off at a predetermined cut-off frequency. For this reason it is advantageous not to have any solid dielectric tuner of greater length than is necessary.

The attenuation :r of a cut-off circular guide in decibels per unit length at any frequency .f corresponding to a wavenlength i( is circular guide must be low. It should therefore experience an attenuation of the order of 30 to 40 decibels. We may conclude from (8) that with the assumption n should have a minimum value of 2 to 3.

Fig. 4 illustrates the application of the open end resonator to a reflex type oscillator. The coaxial resonator consists of an inner tubular conductor 25 and an outer tubular conductor26. These coaxial conductors are supported at their lower ends by the dielectric envelope of a ldischarge device 2 of the repeller type having a cathode heater 28, cathodeV 29, beam electrode 33, grids 3i and 32 and a repeller electrode 33. The grids 3| and 32 are centrally aligned with the coaxial conductorsA 26 and 25 respectively. When the cathode is energized in the usual mannerl and the grids 3l and 32 are polarized to a high positive potential by the source 34, the electron gun constituted by the cathode beam electrode and grid 3l will impel a beam of electrons through the grids 3l and 32 toward the repeller 33.Y With the repeller 33 Vpolarized over the conductor 35 to a negative potential with reference to the cathode, the device will operate as described in the Pierce article to which reference has been made to produce oscillations of a frequency primarily determined by the resonant frequency of the resonator. andsecondarily by the transit time of the electrons in theretarding eld'which is in vturn a function of -thexspacing of the repeller 33 and 'grid 32 and of the potentals of this. grid"l andi oft. the "repeller electrode: The'coaxial structure 25; 26 vis-designed tothave a natural fundamental resonance at^the" frequency,l of the desired oscillations. As will vbe apparentthe outer conductor` 26Y is "extendedasat 26 Jan.additional distance s vvsuiiicient to constitute a cylindrical wave guide'havingalow pass-cut-off inexcess of the oscillationfrequencysoasto'precldej loss l of oscillation energy`- from the `ropenA en'dlof the .coaxial resonator.

Inl order to provide'anyoscillation output cir= cuit; a coaxial v:conductor system'31, 38 is connected vtothejresonatorat'an aperture at one 'side offthe cylinderV 26j. the inner tubular conductor 3Vpassing, through the apertureancl Aconnecting electrically withjthe innerconductor 251of `the resonatorwhil'e. the` outer' conductor 38 'is' connectedfelectrically tothe outer conductor 26 of th'ebresonator. Thepolarizing lead [for the repellr 33jpasses outithroughthe inner tubular memberA STand'leavesit-by way of a coaxial stub circuit '.39 which may' be designed in accordance withwellknown principles to act' as a shunt choke c-ircuittlia-t will'not withdraw'oscillation.

energylv from .the output coaxial 31,'.381

'ITuningiof theoscillatormay be effected by motionfin either longitudinal direction, indicated by the .double arrow, ,of a ring .4U-"of dielectric material encompassing,.th'einner conductor 25. This ringA has the.. eict'. of altering the electrical lengthV of the ycoaxial resonator without, however, permitting thenatural frequency. thereof vto extend into theIange abovethe .cut-ofl'of the cylindricalwave guide.v To manipulate the ring, stubs .4| Aof dielectric. ysubtsance extending laterally from.. and -integral with the ring 40 may projectthrough slots 42 in the wall of 'conductor 26. The stubs may bemoved manually or by any desired tuning' mechanism and maybe .xed in. position after the tuning operation in any Well-known manner. An indicator 43 cooperatingvzwith ya stationary scale44-may be attached to :one of rthestubs toy indicate .the frequency sete ting.

It-will -be yapparent that thev structure of Fig.

4 enables very high oscillation frequencies to be.

attained by the repeller oscillator without reducingthe resonator to ydimensions which from a mechanical standpoint would make the resonator difficult to construct or to' manipulate.

Fig. 5T shows a modification-'of thesystem .of Fig. 4 for producing microwaves.` In this modified system-'the outerand inner coaxial members 44 and 45 constitute the resonator and the extension 46,of outer member'44 lserves as awave guide Yto preventescape of microwave oscillations from theupperopenen'd'of the resonator. A dielectric. tunin'gcylinderv 4l tting between the coaxial members ismounted formotion in an axial 'direction to tune'the oscillator. The'cylinder 41"s carried bya cap plate V48 and plunger rod. 491 both also comprising dielectric material andljthe rod 49 may have'anndicating `pointer 50 attached thereto' to indicate the frequency on aflxed scale 5|. Negative'biasing potential for the. repeller'53 is derived over a'path 54 extending by way 'of th'ecentral insulated conductor 55 which, associated with the outer tubular conductor 56 having centrally in-turned anges 51 at its outer end, constitutes a quarter wavelength short-circuited line at the central microwave frequency desired and hence serves as a high impedance choke preventing the escape of microwave energy over the path 54. The conducting sleeve surrounding but insulated from theconductor'tserve'las the central orfin-ner" elementi of the `coaxial high frequency choke--is connected-to Athe-'inner coaxial member #i5-of the oscillator resonator atta point such "that for thej mid-'band-frequency the" radial -iieldisesubstantially= zero. Consequentlyfv atth-is frequency there* is -n'o tendency A'for f the leads fto sa-p high' frequencyienergy from thelfoscillator: With fde parture` from the mid-band lfrequency this f deal condition' 'not longer exists but -the coaxial *choker eiectively serves 4to' suppress transmission of high frequency-power over the rangeof 'frequencies ofinterest.vv Microwave 'output `energy may-be sup-` plied Abyj-the/'oscillator over aV coaxial outputcircuit '58),' 59 coupled b'y-y a loop aliuto the internal electromagneticfield ofthe coaxiall resonator;

The principle of an open-circuited resonatornotonly enables the-mechanical structure-to l-be made largeras-"hasbeen' indicated butv itA also makes `for other ymechanical simplification; For example,V if the-extension 46 be -suciently-long so nthat the'cut-offwave guide section introduces:4 anextremely highimpedance `rbetween vthe outerv end '6I 4of the wave guidera-ndthe--end162 ofthe coaxial resonator-1th'enature-of the outeri termination-of the wave-guide section-at 6| is yof little* moment froml an electrical 'l standpoint from the nviewpoint of thecoa-xial resonatorv which remains `'open-lcir'cui-ted; ItE is "accordingly feasible 'to close' "that end Aof the Iwave guide 4 6 -with ai* metallic 'cap-62 which'tmay. cooperate to'sup-A portk and guide the"-dielectricl plunger "49;

It'will lbe `understood that `in the structures -of both Figs. 4 and 5 the open ended portion-oi the "coaxial resonator 'is so designed as'to `comprise substantially a quarter wavelength struc;V ture attthe vvmid-frequency; or" *the* vdesired range. The'` end' fof-the l`r`resonator'to ywhich the' Ielectron dischargey tube electrodes-'are connected is likewise ai quarter Vwavelength lresonator mechanically.`= shortened to take 'into account the i' loadingi 'imposed byI the interelectrode capacitances. Then'etoverlall length of thel coaxial resonator isv therefore somewhatv less than thef half' wavelength ideal structure of Fig.'v l butv like thatof Fig." 2 is excessrof a" qua-rter "wavelength Fig." 6 illustratesan vembodiment ofthe invention in'which `th'e'resonator and the electron dischargedevice' are'. contained `within' a; metallic container64.' The input circuit; output circuit and the' electron gun" structurefmay'fbe identical in-1 principle withthe'corresponding elements of tlieFi'g'-5 apparatus:- I-Ioweverf' the gridst and 6l which together constituterthe"interactionrgaproL tlie\^elt=.^ctron-dischargev device are diffrentlyi supported by the=metallic container.` Grid 66 carriedv :by -aI plate '69 extending ='across th-einterirsurface-'of the containertil 'and serv# igto" electricallyfclose#thex lower`V end ofi' the coaxiatresonator.A Grid? 6T is carried atthe lower* endof the Iinner' coaxial member l 6 which infturn isf'supported by a spider=structure- Tlf consisting of 'F'radi'alf arms of dielectric material as is f better 'showzr infv -Fige 7.*- Tlie spider-f structure l l fis-"attached to and 'support-ed by facentral 'column-`112 l'orro'd of E'dielectric 'material `which in turn is attached at its upper end and supported by the end plate 13 which serves to close the upper end of the evacuated container 64. Current leads for the electron gun pass out through beads of insulating material 14 and 'I5 in the usual manner.

Tuning of the oscillator of Fig. 6 is. effected by varying the length and consequently the transit time of the interaction space between the grids 66 and 61 and, what is still more im-l portant, the capacitance between grids 66 and 6 1. This may be accomplished by varying the position of the tuning screw 16 which is supported in the bracket 11 above the end plate 13. The end plate 13 is provided with a folded or reticulated bellows portion 18 thus permitting the central part of the plate 73 to bev flexed upwardly and downwardly in response to the motion of the screw 16 so as to carry with it the rod 12 of dielectric material and the attached structure consisting of the inner coaxial member 'l0 and the upper grid 51. The conducting choke arm 19 which surrounds the repeller lead conductor and serves to connect coaxial member 10 with the outer coaxial member 64 may be made sumciently exible to permit motion of member 10 and its grid 67 toward and away from the grid 66 as determined by the adjusting screw 16. As in the case of the choke of Fig. 5 the connection of the inner coaxial member of the choke to the inner coaxial member of the resonator is preferably at a point of minimum radial field at the mid-band frequency.

Fig. 8 illustrates a modification of the upper portion of the structure of Fig. 6 in which in order to increase the low cut-off frequency of the'high-pass wave guidestructure, the diam'- eter of this wave guide structure is reduced. The reduced diameter portion includes in its cylindrical vertical walls a bellows 80 permitting tuning in the same manner as in the structure 0 Fig. 6.

The mode ofthe desired oscillation is the dominant mode of the coaxial resonator in which the electric vectors extend radially between the inner and outer members of the coaxial struc-.

ture. This desired mode tends to impress at the contiguous end of the cut-off wave guide oscillations of the TMol mode as illustrated for example at the top of page 473, Ultra-High Fre'- quency Techniques, Brainerd, Wheeler, Reich and Woodruff, D. Van Nostrand Company, Incorporated, 1942.

Although the invention has been illustrated and described as embodied in a reflex oscillator it will be apparent that it is capable of 'incor poration in various combinations utilizing coaxial resonators and particularly in such combinations in which the coaxial resonator is coupled to the electrodes of an electron dischargek device to serve as a frequency selective system' with a high Qat high microwave frequencies.

What is claimed is: l. A resonator comprising a section of coaxial conductor, an electron discharge device having two perforate electrodes between which a high frequency oscillation potential exists connected respectively to the two conductors of the coaxial section near one end, the other end of the section being electrically open and the outer coaxial conductorbeing prolonged at its open end beyond the corresponding end of the inner coaxialconductor to constitute a cylindrical hollow pipe wave guide, the length and diameter of the said hollow pipe wave guide being such as to cause the guide to present substantially negligible admittance at the frequency of oscillation for which the most fundamental resonance mode of the coaxial section occurs.

2. A reflex oscillator comprising a dielectric envelope, an electron gun and a repeller electrode therein, two coaxial conducting tubes fixed in position with respect to and supported by said envelope, each of said tubes having one end closed by a grid aligned between the gun and the repeller and the other end open, the outer of the tubes section near one end, the other end of the section being electrically open and the outer coaxial conductor being prolonged at its open end a length equal to at least two times the inside diameter of said outer coaxial conductor to constitute a hollow pipe cylindrical wave guide and to minimize power loss at the said open end.

4. A refiex oscillator comprising an electron discharge device having an electron emitter, a pair of electron permeable electrodes and a repeller aligned in position with said electron emitter, a coaxial resonator having inner and outer conducting members respectively connected to said electron permeable electrodes, the outer member being longer than the inner so as to extend beyond it at the end remote from its connection to the permeable electrodes by a length equal to at least two times the inside diameter of said outer member.

5. A reflex oscillator in accordance with claim 4 and having in addition means for varying the effective electrical length of the said inner member to Vary the natural resonance frequency of the coaxial resonator.

y WILLIAMv Gr. SHEPHERD.

,REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS NumberI Name Date 2,157,855 Koch May 9, 1939 2,197,123 King Apr. 16, 1940 2,218,223 Usselman et al Oct. 15, 1940 2,300,315 Puhlmann Oct. 27, 1942 2,376,785 Y Krasik May 22, 1945 2,406,850 Pierce Sept. 3,1946 2,421,591 Bailey June 3, 1947 2,438,832 Turner Mar. 30, 1948 2,439,388 Hansen Apr. 13, 1948 2,464,801 Gardner Mar. 22, 1949 2,487,547 Harvey Nov. 8, 1949 2,514,544 Hansen July 11, 1950 

